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Extend discovered checks regardless of SEE

A simple idea, but it makes sense: in current master the search is extended
for checks that are considered somewhat safe, and for for this we use the
static exchange evaluation which only considers the `to_sq` of a move.
This is not reliable for discovered checks, where another piece is giving
the check and is arguably a more dangerous type of check. Thus, if the check
is a discovered check, the result of SEE is not relevant and can be ignored.

STC:
LLR: 2.96 (-2.94,2.94) [0.50,4.50]
Total: 29370 W: 6583 L: 6274 D: 16513
http://tests.stockfishchess.org/tests/view/5c5062950ebc593af5d4d9b5

LTC:
LLR: 2.95 (-2.94,2.94) [0.00,3.50]
Total: 227341 W: 37972 L: 37165 D: 152204
http://tests.stockfishchess.org/tests/view/5c5094fb0ebc593af5d4dc2c

Bench: 3611854
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miguel-l authored and snicolet committed Jan 29, 2019
1 parent ff97a9f commit 9050eac59564fe96b3f24d2889bbef7336b28100
Showing with 5 additions and 3 deletions.
  1. +5 −3 src/search.cpp
@@ -953,11 +953,13 @@ namespace {
else if (cutNode && singularBeta > beta)
return beta;
}
else if ( givesCheck // Check extension (~2 Elo)
&& pos.see_ge(move))

// Check extension (~2 Elo)
else if ( givesCheck
&& (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
extension = ONE_PLY;

// Extension if castling
// Castling extension
else if (type_of(move) == CASTLING)
extension = ONE_PLY;

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